An anchoring member of this type, a so-called fixture, is previously known by EP 0 237 505. In this case the cavities are formed from two perpendicular through holes which are perpendicular to the longitudinal axis of the fixture or by three not through-going cavities formed on the outer, circular surface of the anchoring member so that cutting edges having a positive cutting angle are formed.
The advantage of a self-tapping fixture is the fact that the fixture can be more easily installed in the jaw-bone. In the normal procedure for installing a fixture a hole is drilled in the jaw-bone. Then drills with successively increasing drilling diameters are used until the hole diameter corresponds to the root diameter of the threaded fixture. In the normal surgical method indicated by Dr Br.ang.nemark a screw tap is used to form the internal thread into which the threaded part of the fixture is inserted. When using a self-tapping fixture of the above-mentioned type the installation can be carried out without any screw tap.
The hardness of the bone (jaw-bone) into which the fixtures are installed varies to a big extent. Some patients have a very thin outer bone layer, corticalis, which is hard, but the rest of the bone, the inner spongious bone, is very soft. For some patients, on the other hand, all the bone through the entire section is hard.
Self-tapping fixtures have previously been used primarily for softer bone-types, i.e. bone in the over-denture. Self-tapping fixtures for harder bone types must have very good cutting characteristics in order to limit the torsional force so that the fixture itself or the surrounding bone tissue will not be damaged. Very hard requirements on the fixture design as well as on the cutting sharpness then must be fulfilled.
In another, more simple method for using a self-tapping fixture the hole in the jaw bone is drilled big enough to allow only the outer part of the threading to contact the bone. However, this is a risky method as the initial stability of the fixture in the bone is reduced.